You should probably get atleast a 2.5g to start with. Anything smaller requires too much maintenance. But should get the biggest you can afford.

Another tip, Im assuming your getting your betta from a petstore, dont get one of the biggest bettas you can find, normally the bigger ones are older and therefore closer to death, get one of the younger ones so it has a few years on it still. Also watch out for missing fins, tails, or scale unless your planning on"rescuing" one. Look for the most active ones swimming around their cup, and if your at Petco, the ones in the blueish water arenormally the newest so they are probably healthier.

It is a mass of bubble the male will blow on top of the water, it is a sign that to females that hes ready to mate but it is also a sign to us that he is happy ans healthy. But just because there is no bubblenest doesnt mean they're not healthy